Optical drives often employ radio frequency (RF) oscillators to reduce laser noise. FIG. 1, shows the block diagram of a laser driver 102 that is used to drive a laser diode 120.
The laser driver 102 includes a write driver 104, a read driver 106, and an oscillator 108. The write driver 104 is capable of providing different write current signals to the laser diode 120, which are useful for writing data to a storage medium (e.g., a DVD or CD). The read driver 106 is capable of providing different read current signals to the laser diode 120, which are useful for reading data from the storage medium. It is common for the laser diode 120 to mode-hop due to external light reflected back into the laser cavity and generate additional system noise. To avoid or reduce the mode-hopping, an RF current can be applied to the laser diode 120 using oscillator 108. The oscillator 108 is most often used in this manner when in read mode, resulting in the current signal generated by the oscillator 108 being added to the current signal generated by the read driver 106, with the resulting current signal driving the laser diode 120.
The oscillator 108 pumps significant current into the laser diode 120. Harmonics generated by the oscillator propagate down traces of the board on which the laser driver is mounted and often cause problems with electromagnetic interference (EMI) emissions.
One solution for reducing harmonics is to produce an oscillator that generates a substantially pure sine wave. However, generating a pure sine wave is difficult because of inherent non-linearities of circuit elements. Any distortion arising from these non-linearities result in harmonics at the output.
Another solution for reducing harmonics is to filter the output of the oscillator with a passive network that includes resistors, capacitors and inductors. However, such a passive filter is difficult to implement at RF frequencies due to the sharp cutoff required between the fundamental and the harmonics. Furthermore, the inductors required occupy a large amount of chip real estate, and the resistors of a passive filter would dissipate power, neither of which effect is desirable.
Accordingly, it would be beneficial to provide oscillators and waveforms that overcome some, and preferably all, of the above disadvantages.